The combustion of solid propellants is a progressive phenomenon localized on the surface of the propellant grain. The burning rate, assuming homogeneous ignition, is defined as the distance traveled per second by the flame front perpendicularly to the exposed surface of the grain.
The burning rate is dependent upon the pressure of the surrounding gas phase. The relationship may be expressed r = K .times. P.sup.n wherein r is the burning rate, K is a proportionality constant, P is the absolute pressure and n is the pressure exponent. It is apparent that when n is positive increase in pressure will lead to increased burn rate and that the greater n is, the greater will be the increase in r for a given rise in P.
A propellant with a high burning rate expells a larger amount of gases in a given period of time than a slower burn rate propellant. The result is a higher mass flow rate to perform a desired function.
A catalyst is frequently used to transform a slower burning propellant into a faster burning one. A wide variety of catalytic materials are known to be useful for control of burning rate. Typical of these are materials such as iron oxide, ferrocene, copper oxide, copper chromite, various organometallic compounds, carborane and various carborane derivatives.
It is frequently advantageous to reduce the pressure exponent of a propellant so as to reduce the fluctuation in pressure caused by a change in burn rate induced, for example, by irregularity in manufacture of the propellant grain. A low pressure exponent normally is indicative of a low temperature sensitivity characteristic, and therefore has less effect on pressure with changes in temperature where the burning is conducted in a combustion chamber from which the combustion products are exhausted, as in a rocket.
While none of above mentioned burn rate catalyst are known to have the ability to also reduce the pressure exponent at high pressures (&gt; 2000 psia), the catalyst of the instant invention possesses this property in both aluminized and non-aluminized solid composite propellants.
The tailoring of burning rate and physical properties in a propellant based on ammonium perchlorate but without metallic fuel such as aluminum powder is not difficult. When such propellants are tested in full scale rocket motors it is difficult to avoid combustion instability. The susceptibility of these propellants to such instabilities, commonly seen as oscillations in pressure thrust-time traces recorded during the combustion of a propellant, is most acute at high burning rates, and high test temperatures, although there are some exceptions.
Boosting propellant performance with powdered aluminum lends stability in that it dampens such oscillations. Such metal containing propellants burn with the evolution of copious amounts of smoke largely due to formation of metal oxides. Despite their inherent combustion instability metal free ammonium perchlorate propellants have the virtue of being relatively smokeless, except for HCl clouds.
Propulsion with low or zero smoke has become of increasing importance in a number of tactical weapons system. Excessive quantities of smoke produced by a propellant can interfere not only with weapons guidance, but in air launch operations, with pilot visibility in general. Smoke can also assist detection by the enemy in field operations.
The inclusion of catalytic amounts of finely divided aluminum oxide in metal free propellants containing inorganic perchlorate as the oxidizer permits the control of combustion instability while retaining the smokelessness of the combination.